How were "clusters of galaxies" formed?
The structure of clusters of galaxies are apparently formed under static equipartition between the pressure of hot plasma and the gravity caused mainly by dark matter. This seems to be a strange problem in kinematics. Because it radiates X-rays, the hot plasma is gradually cooled. The cooled plasma is getting denser in the central region, and is thus more effective in radiation and cooling. Since it makes the plasma cooler, this is a positive feedback process. Thus, the hot plasma in clusters of galaxies are expected to cause gravitational core corruption in 100 million to 1 billion years. However, clusters of galaxies are stable for 10 billion years, and we see no sign of corruption.
So, what in the cluster of galaxies compensates for the inevitable energy loss and prevents this corruption? Candidates proposed are heat conductance from the core surrounding hot plasma, dissipation of kinetic energy of galaxies, or heating by hot outflow plasma from the central supermassive black hole (active galactic nucleus). To solve this problem, the key is temperature and velocity of the hot plasma. The high resolution X-ray spectroscopy provided by XRISM will enable us to determine the temperature and velocity distribution in the plasma of a cluster of galaxies, and leads us to understand the hidden dark matter distribution.